The present invention relates generally to a lifting device, and more particularly to an oil pressure device of a jack.
As shown in FIG. 1, a prior art jack comprises an oil pressure cylinder 1 which is provided with a first receiving space 2, a second receiving space 3, and a third receiving space 4 disposed on the cap cover located at one end of the oil pressure cylinder 1. The third receiving space 4 is provided therein with a push rod 5. As the push rod 5 is pulled backwards, the oil kept in the second receiving space 3 is forced to flow into the third receiving space 4 via a one-way valve 6. The oil in the third receiving space 4 is forced to flow into the first receiving space 2 at the time when the push rod 5 is caused by an external force to move forward, thereby causing a driving rod 7 to move forward so as to provide the jack with a power for lifting or hoisting a heavy object. The second receiving space 3 is provided therein with an oil unloading rod 8, which can be caused to turn to withdraw from an oil returning oil 9. As the pressure in the first receiving space 2 begins to diminish, the driving rod 7 is caused to withdraw by the reaction force of the heavy object lifted by the jack, thereby causing the oil to flow back to the second receiving space 3 from the first receiving space 2 via the oil returning hole 9.
The prior art oil pressure device described above is defective in design in that the driving rod 1 is caused to move forward slowly and that the jack can not be therefore raised rapidly to reach a heavy object intended to be lifted.